leveldb: new .gitignore entry

Signed-off-by: Sage Weil <sage@newdream.net>

diff --git a/src/leveldb b/src/leveldb
index 8b5ada5c89c0fc34b3c277337d9b3f2c6f1fb85d..a5fb0fe5a0e308824f4bccece12e879c8c8a6318 160000 (submodule)
--- a/src/leveldb
+++ b/src/leveldb
@@ -1 +1 @@
-Subproject commit 8b5ada5c89c0fc34b3c277337d9b3f2c6f1fb85d
+Subproject commit a5fb0fe5a0e308824f4bccece12e879c8c8a6318